The present invention relates generally to services in relation with communication via telephone networks, more particularly in relation with the service known as Caller ID or Call Waiting (CIDCW, CID type 2), especially on PSTN lines, and concerns a telephone or a similar communication terminal with improved calling alert signal (CAS) detection.
In telephone networks offering a Caller ID or Call Waiting or CIDCW service, the following steps are executed by the local telephone exchange station or central office an the considered telephone or similar communication terminal, upon arrival at said station or office of each new call or connection request from a new caller for said telephone or communication terminal while the latter is already engaged and in communication with a distant telephone or communication terminal of a calling or called user:
the station or office mutes or cuts off the transmission from the engaged distant user;
the station or office sends a call signal (SAS, generally at 440 Hz) and then a calling alert signal (CAS, generally a bi-frequency signal at 2130 Hz and 2750 Hz during 80 ms);
the considered telephone or communication terminal detects the CAS signal, mutes its sending path and send an acknowledgement signal (for example DTMF xe2x80x9cDxe2x80x9d) back to the station or office;
upon reception of the acknowledgement signal, the station or office transmits identification data (name, phone number, . . . ) concerning the new caller which is stored and/or displayed by the considered telephone or communication terminal after setting its modem (for example V23 or Bell 202) into receiving mode;
and finally the station or office suspends the cut off of transmission from the engaged distant user, the considered telephone or communication terminal demutes its sending path and the existing audio path is restored and the communication can be continued normally as before interruption.
The main difficulty encountered while providing this service is the reliability of the detection of the CAS signal, especially as CAS signals have to be detected while speech is present, i.e. while voice signals are exchanged between the two engaged telephones or similar terminals.
Indeed, speech can cause non detection of really existing CAS signals, but can also generate false detection, i.e. positive detection of non existing CAS signals.
Now, as the considered telephone or terminal has to mute its sending path and its receiving path (modem set into receiving mode) when it assumes to have detected a CAS signal, false detection has of course dramatical effects, by muting the conversation for seconds.
In order to verify the false detection rate of terminals, so-called xe2x80x9ctalkoffxe2x80x9d and xe2x80x9ctalkdownxe2x80x9d testings have been specified (see BELLCORE: GR30 core, ST-TSV-002476 and SE-3004) and, in order to keep false detection rates under bearable threshold values, high demanding criteria have been proposed as to reliability of CAS signal detection when mixed with speech, which are difficult to reach even with sophisticated detection systems.
A xe2x80x9ctalkoffxe2x80x9d error occurs when the office or station has not sent any CAS signal while the telephone or terminal is convinced to have detected a CAS signal in the speech signal produced by the near end or local speaker (user of the local concerned telephone or terminal) or by the distant or far end user (user of the telephone or terminal in communication with the local user).
A xe2x80x9ctalkdownxe2x80x9d error arises when the office or station has actually sent a CAS signal which is then not detected by the concerned telephone or terminal because, when detection is performed, it has been masked, distorted, interfered with or otherwise modified, mainly by the speech signal of the local user.
It is therefore a main effect of the present invention to propose a simple and low cost solution which allows to decrease significantly, or even to remove completely, all xe2x80x9ctalkoffxe2x80x9d problems generated by far end speech and to greatly simplify the CAS signal detection process. Furthermore, it should be easy to add the proposed solution to any existing CAS detection system, to improve the performances of the latter and to integrate it in any analogue or digital telephones or communication terminals.
Therefore, the present invention concerns a telephone or communication terminal comprising a receiving path and a sending path linked to the communication line, the receiving path being connected to a system able to detect, during a communication, an alert signal or CAS signal sent by the local telephone exchange station or central office upon any new call or connection request for the considered telephone or terminal and allow reception by the latter, for example, of data related to each new caller, by interrupting temporarily the presently existing communication, characterized in that it comprises a detector means able to detect any activity or any signal transmission and/or the absence of the same, in the sending path of the considered telephone or terminal and to deliver a corresponding information signal (IS) to the CAS signal detection system, the latter adjusting its detection method or mode or selecting a corresponding detection subsystem accordingly.